It's normal but very frustrating! Dd1 was born at 34weeks when I thought she was closer to 36 going by my cycle. Our peadatricion agreed he said she didn't look or act like a 6 week premmie, she was closer to 4 weeks because she was such a fighter and got home in three weeks. He expected her to be in hospital for longer.

The ultrasound moved my date again with dd2 she almost arrived at 35 weeks but I thought I was further along. The same has happened again, so we are planning for a third early arrival. In my case it gives me a little bit more time before they intervene but I don't trust the early scans, I was charting with the first two so I knew when I had ovulated too.